Ensure custom EUI64 token can be written with RCP firmware

[puddly]

Writing a custom EUI64 via NV3 can fail with RCP firmware. Existing unit tests didn't cover this specific combination, unfortunately.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b50af94) 99.83% compared to head (7ad6bde) 99.83%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #583 +/- ## ======================================= Coverage 99.83% 99.83% ======================================= Files 68 68 Lines 4871 4874 +3 ======================================= + Hits 4863 4866 +3 Misses 8 8 ``` | [Files Changed](https://app.codecov.io/gh/zigpy/bellows/pull/583?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zigpy) | Coverage Δ | | |---|---|---| | [bellows/ezsp/\_\_init\_\_.py](https://app.codecov.io/gh/zigpy/bellows/pull/583?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zigpy#diff-YmVsbG93cy9lenNwL19faW5pdF9fLnB5) | `99.40% <100.00%> (+<0.01%)` | :arrow_up: |

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